What are allergies and how do they affect cats?
One of the most common conditions affecting cats is allergies. In the allergic state, the cat's immune system "overreacts" to foreign substances (allergens or antigens) to which it is exposed. These overreactions are manifested in three ways. The most common is itching of the skin, either localized (one area) or generalized (all over the cat). Another manifestation involves the respiratory system and may result in coughing, sneezing and/or wheezing. Sometimes there may also be an associated nasal or ocular (eye) discharge. The third manifestation involves the digestive system, resulting in vomiting or diarrhea.
Is there more than one type of allergy?
Yes, there are four known types of allergies in the cat:
  • Contact
  • Flea
  • Food
  • Inhalant 
Each of these have some common expressions in cats as well as unique features.
Contact Allergy
Contact allergies are the least common of the four types of allergies. They result in a local reaction to the skin. Examples of contact allergy include reactions to flea collars or types of bedding, such as wool. If the cat is allergic to such substances, there will be skin irritation and itching at the points of contact. Removal of the contact irritant solves the problem. However, identifying the allergen can require some detective work.
Flea Allergy
Flea allergies are common in cats. A normal cat experiences only minor irritation in response to flea bites, often without any itching. The flea allergic cat, on the other hand, has a severe, itch-producing reaction when the flea's saliva is deposited in the skin. Just one bite causes such intense itching that the cat may severely scratch or chew itself, leading to the removal of large amounts of hair. There will often be open sores or scabs on the skin, allowing a secondary bacterial infection to begin. The area most commonly affected is over the rump (just in front of the tail). In addition, the cat may have numerous, small scabs around the head and neck. These scabs are called miliary lesions, a term that was coined because the scabs look like millet seeds.
The most important treatment for a flea allergy is to get the cat away from all fleas. Therefore, strict flea control is the backbone of successful treatment. Unfortunately, this is not always possible in warm and humid climates, where a new population of fleas can hatch out every 14 to 21 days. However, many flea products are able to kill fleas before they have a chance to bite your cat. When strict flea control is not possible, injections of corticosteroids can be used to block the allergic reaction and give relief. This is often a necessary part of dealing with flea allergies. Fortunately, cats appear relatively more resistant to the side-effects of steroids than other species. If a secondary bacterial infection occurs, appropriate antibiotics must be used. 
Inhalant Allergy
The most common type of allergy is the inhalant type, or atopy. Cats may be allergic to all of the same inhaled allergens that affect humans. These include tree pollens, grass pollens, weed pollens, molds, mildew and dust mites. Many of these allergies occur seasonally, such as ragweed, cedar and grass pollens. However, others are with us all the time, such as molds, mildew and dust mites. When humans inhale these allergens, we express the allergy as a respiratory problem sometimes called "hayfever." The cat's reaction, however, usually produces severe, generalized itching. 
Most cats that have an inhalant allergy are allergic to several different allergens. If the number of allergens is small or the cat has seasonal allergies, itching may last for just a few weeks at a time during one or two periods of the year. If the number of allergens is large or the allergies are present year-round, the cat may itch constantly. 
Treatment depends largely on the length of the cat's allergy season and involves two approaches. Steroids will dramatically block the allergic reaction in most cases. These may be given orally or by injection, depending on the circumstances. As previously stated, the side effects of steroids are much less common in cats than in people. If steroids are appropriate for your cat, you will be instructed in their proper use.
Some cats are helped considerably by a hypoallergenic shampoo, as it has been demonstrated that some allergens may be absorbed through the skin. Frequent bathing is thought to reduce the amount of antigen exposure through this route. In addition to removing the surface antigen, bathing alone will provide some temporary relief from itching and may allow a lower dose of steroids.
The second major form of allergy treatment is desensitization with specific antigen injections, or allergy shots. Once the specific sources of the allergy are identified, very small amounts of the antigen are injected weekly. This is all in an attempt to reprogram the body's immune system. It is hoped that as time passes, the immune system will become less reactive to the problem-causing allergens. If desensitization appears to help the cat, injections will continue for several years. For most cats, a realistic goal is for the itching to be significantly reduced in severity; in some cats, itching may completely resolve. Steroids are not used with this treatment protocol, except on an intermittent basis. This therapeutic approach is recommended for the middle-aged or older cat that has year-round itching caused by an inhalant allergy. 
Although desensitization is the ideal way to treat an inhalant allergy, it does have some drawbacks and may not be the best choice in certain circumstances. 
  • Cost: This is the most expensive form of treatment. 
  • Age of Patient: Because many cats develop additional allergies as they get older, young cats may need to be retested one to three years later.
  • Success Rate: About 50 percent of cats will have an excellent response to desensitization. About 25 percent have a partial to good response while another 25 percent get little or no response. The same statistics are true for people undergoing desensitization.
  • Food Allergies: Although tests for food allergies are available, the reliability of the test is so low that it is not recommended at this time. A food trial remains the best diagnostic test for food allergies.
  • Response Time: The time until apparent response may be two to five months or longer. 
  • Interference of steroids: Cats must not receive oral steroids for two weeks or injectable steroids for six weeks prior to testing; these drugs will interfere with the test results.
Food Allergy
Cats are not likely to be born with food allergies. More commonly, they develop allergies to food products they have eaten for a long time. The allergy most frequently develops in response to the protein component of the food; for example, beef, pork, chicken or turkey. Food allergies may produce any of the clinical signs previously discussed, including itching, digestive disorders and respiratory distress. 
We recommend testing for food allergies when the clinical signs have been present for several months, when the cat has a poor response to steroids or when a very young cat itches without other apparent causes of allergy. Testing is done with a special hypoallergenic diet. Because it takes at least eight weeks for all other food products to get out of the system, the cat must eat the special diet exclusively for eight to 12 weeks or more. If the diet is not fed exclusively, it will not be a meaningful test. We cannot overemphasize this. If any type of table food, treats or vitamins are given, these must be discontinued during the testing period. 
Because cats being tested for inhalant allergies generally itch year-round, a food allergy dietary test can be performed while the inhalant test and antigen preparation are occurring. 
See Canine Allergies